Normally cylinder plugs of locks are retained in a cylinder plug housing by, inter alia, a retaining ring disposed in a retaining ring groove on the cylinder plug stem. When it is desired to change the cylinder plug it is first necessary to remove the retaining ring from the retaining ring groove. The present invention provides a simple and easy to use hand tool for removing the retaining ring from the retaining ring groove in the stem of a cylinder plug so that the cylinder plug can be removed from its housing. The tool of the instant invention can also be used to insert the retaining ring into the retaining ring groove.